Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 30 October 2018 (MN LATEX style file v2.2) An anomaly detector with immediate feedback to hunt for planets of Earth mass and below by microlensing M. Dominik,1⋆† N. J. Rattenbury,2 A. Allan,3 S. Mao,2 D. M. Bramich,4 M. J. Burgdorf,5 E. Kerins,2 Y. Tsapras,5 and Ł. Wyrzykowski6,7 1SUPA, University of St Andrews, School of Physics & Astronomy, North Haugh, St Andrews, KY16 9SS, United Kingdom 2Jodrell Bank Observatory, Macclesfield, Cheshire, SK11 9DL, United Kingdom 3School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom 4Isaac Newton Group of Telescopes, Apartado de Correos 321, 38700 Santa Cruz de La Palma, Canary Islands, Spain 5Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, United Kingdom 6Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom 7Warsaw University Astronomical Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland 30 October 2018 ABSTRACT The discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet, impressively demonstrated the sensitivity of the microlensing technique to extra-solar planets below 10 M⊕. A planet of 1 M⊕ instead of the expected 5 M⊕ for OGLE 2005-BLG-390Lb (with an uncertainty factor of two) in the same spot would have provided a detectable deviation with an amplitude of ∼ 3 per cent and a duration of ∼ 12 h. While a standard sampling interval of 1.5 to 2.5 hours for microlensing follow-up observations appears to be insuffi- cient for characterizing such light curve anomalies and thereby claiming the discovery of the planets that caused these, an early detection of a deviation could trigger higher-cadence sam- pling which would have allowed the discovery of an Earth-mass planet in this case. Here, we describe the implementation of an automated anomaly detector, embedded into the eSTAR system, that profits from immediate feedback provided by the robotic telescopes that form the RoboNet-1.0 network. It went into operation for the 2007 microlensing observing season. As part of our discussion about an optimal strategy for planet detection, we shed some new light on whether concentrating on highly-magnified events is promising and planets in the ’resonant’ angular separation equal to the angular Einstein radius are revealed most easily. Given that sub-Neptune mass planets can be considered being common around the host stars probed by microlensing (preferentially M- and K-dwarfs), the higher number of events that arXiv:0706.2566v1 [astro-ph] 18 Jun 2007 can be monitored with a network of 2m telescopes and the increased detection efficiency for planets below 5 M⊕ arising from an optimized strategy gives a common effort of current mi- crolensing campaigns a fair chance to detect an Earth-mass planet (from the ground) ahead of the COROT or Kepler missions. The detection limit of gravitational microlensing extends even below 0.1 M⊕, but such planets are not very likely to be detected from current cam- paigns. However, these will be within the reach of high-cadence monitoring with a network of wide-field telescopes or a space-based telescope. Key words: planetary systems – gravitational lensing – methods: observational. 1 INTRODUCTION The discovery of OGLE 2005-BLG-390Lb (Beaulieu et al. 2006; Dominik et al. 2006), estimated to be 5 times more massive than After Mao & Paczy´nski (1991) first pointed out that microlens- Earth, with an uncertainty factor of two, under the lead of the ing events can be used to infer the presence of extra-solar plan- PLANET (Probing Lensing Anomalies NETwork)/RoboNet cam- ets or place limits on their abundance, this technique has now paign demonstrated that microlensing not only can detect massive become established with several claimed detections (Bond et al. gas giants, but also planets that harbour a rocky/icy surface under a 2004; Udalski et al. 2005; Beaulieu et al. 2006; Gould et al. 2006). thin atmosphere. Moreover, it provided the first observational hint that cool rocky/icy planets are actually quite common, as previ- ⋆ Royal Society University Research Fellow ously predicted by simulations based on core-accretion models of † E-mail: [email protected] planet formation (Ida & Lin 2005). c 0000 RAS 2 M. Dominik et al. It was already estimated by Bennett & Rhie (1996) that there and moreover, it gave PLANET a reliable chance to detect planets is a non-negligible chance of 1–2 per cent for detecting an Earth- of a few Earth masses provided that these are not rare around the mass planet located at about 2 AU from its host star by means of stars that cause the microlensing events. The discovery of OGLE observing a few-per-cent deviation in a microlensing light curve. 2005-BLG-390Lb (Beaulieu et al. 2006; Dominik et al. 2006) ex- However, such a discovery requires photometric measurements on plicitly proved the sensitivity of the PLANET observations to plan- a few hundred microlensing events, assuming that a fair fraction of ets in that mass range. the host stars are orbited by such planets. Microlensing events are also regularly monitored by the A sufficient number of events can only arise from monitoring MicroFUN (Microlensing Follow-Up Network) team4. However, dense fields of stars. With a probability of 10−6 for a star in the rather than exploiting a permanent network, MicroFUN concen- Galactic bulge being magnified by more than∼ 34 per cent at any trates on particularly promising events and activates target-of- given time due to the bending of light caused by the gravitational opportunity observations should such an event be in progress. Be- field of an intervening foreground star (Kiraga & Paczy´nski 1994), sides 1m-class telescopes, their stand-by network includes a larger and such a microlensing event lasting of the order of a month, one number of small (down to 0.3m diameter) telescopes operated by namely needs to monitor 107 to 108 stars. This was achieved by mi- amateur astronomers, which are well suited to observe the peaks of crolensing surveys like OGLE (Optical Gravitational Lensing Ex- events over which the source star makes a bright target. periment) (Udalski et al. 1992), MACHO (MAssive Compact Halo Since the PLANET network is restricted in its capabilities Objects) (Alcock et al. 1993), EROS (Exp´erience de la Recherche of monitoring 25 per cent of the currently alerted events with d’Objets Sombres) (Aubourg et al. 1993) and MOA (Microlensing the observational∼ requirements, the planet detection rate could be Observations in Astrophysics) (Muraki et al. 1999) with a roughly boosted by using larger (2m) telescopes or clusters of 1m-class daily sampling. Moreover, all these surveys have been equipped telescopes. In fact, such an upgrade is required in order to ob- with real-time alert systems (Udalski et al. 1994; Udalski 2003; tain a sample that allows a reliable test of models of the forma- Alcock et al. 1996; Glicenstein 2001; Bond et al. 2001) that notify tion and evolution of planets around K- and M-dwarfs. RoboNet- the scientific community about ongoing microlensing events. This 1.05 (Burgdorf et al. 2007) marks the prototype of a network of 2m allows to schedule follow-up observations that provide an increased robotic telescopes, not only allowing a fast response time, but also photometric accuracy, a denser event sampling, and/or coverage a flexible scheduling by means of the multi-agent contract model during epochs outside the target visibility from the telescope site provided by the eSTAR project6 (Allan, Naylor & Saunders 2006; used by the respective survey campaign. Allan et al., 2006). eSTAR is a key player in the Heterogeneous The PLANET (Probing Lensing Anomalies NETwork) col- Telescope Networks (HTN) consortium and involved in the IVOA laboration1 established the first telescope network capable of (International Virtual Observatory Alliance) standards process. round-the-clock nearly-continuous high-precision monitoring of If one aims at the discovery of Earth-mass planets, the stan- microlensing events (Albrow et al. 1998) with the goal to detect dard follow-up sampling of 1.5 hrs usually does not produce the gas giant planets and to determine their abundance. For being able amount of data required to characterize the corresponding signals, to detect deviations of 5 per cent, PLANET aims at a 1-2 per cent and with less frequent sampling one even faces a significant risk photometric accuracy. With a typical sampling interval of 1.5 to 2.5 of missing any hint for a deviation from an ordinary microlensing hrs allowing a characterization of planetary anomalies on the basis light curve. However, planets of Earth mass and even below can be of at least 10-15 data points taken while these last, the required ex- discovered by shortening the sampling interval to 10 min once a posure time then limits the number of events that can be monitored. regularly sampled point is suspected to depart from∼ a model light For bright (giant) stars, exposure times of a few minutes are suffi- curve that represents a system without planet. In order to properly cient, so that PLANET can monitor about 20 events each night or trigger such anomaly alerts, all incoming data need to be checked 75 events per observing season, but this reduces to about 6 events immediately, and prompt action needs to be taken within less than each night or 20 events per season for fainter stars, for which expo- 15 min. The amount of data and the required response time for sure times reach 20 min (Dominik et al. 2002). In 1999, MACHO achieving∼ a good detection efficiency for Earth-mass planets are and OGLE-II together provided about 100 microlensing alerts, out however prohibitive for relying on human inspection.